136 FAMILY MILIOLIDA. 



sheath is divided into tolerably regular areolae by sutural lines, which seem of different texture 

 from the rest (fig. 30). Each horizontal row of these areolae corresponds to one annulus ; and 

 the sutural lines which bound it horizontally, divide that annulus from the annuli above and 

 below. Each division marked out by the vertical lines corresponds to the cluster of branches 

 radiating from one of the great junctural canals of the interspace-system ; and the shell- 

 substance which it includes was probably deposited by the extensions of the sarcode-body 

 which passed through those canals. As no such sutures are discernible in sections, however 

 thin, it seems obvious that the substance of the shell at the lines of junction must closely 

 accord with that of the rest, its difference being only brought out by its more ready solubility 

 in dilute acid. I have noticed in some instances that sutural lines are only distinguishable 

 between the successive annuli; the portions of the exogenous sheath belonging to each 

 annulus being entirely homogeneous. 



1 97. It is obvious, from the account now given of the structure of this, the most developed 

 type oiBacfi/Iojjora, that the development of the thick exogenous sheath which is formed around 

 the chambered portion of the cylinder could scarcely have taken_ place from the isolated seg- 

 ments of the sarcode-body contained within its chambers ; but that it must have proceeded 

 from an aggregate mass filling the whole interior of the cylinder. For it appears scarcely 

 questionable that the " interspace-system " of large canals, with their bundles of smaller 

 branches, must have been occupied during life by extensions of the sarcode-body ; since the 

 peculiar structure of the exogenous sheath cannot be probably accounted for in any other 

 way, than by supposing each constituent portion of it to have been built up by the agency of 

 one cluster of such extensions ; each of these clusters remaining as distinct from those adjacent 

 to it, as are the segments occupying the cavity of the chambers themselves. But it is the 

 distinctive peculiarity of this " interspace-system," that it has extremely little connection 

 with the chambers ; in this respect differing widely from the true " canal-system " of Calcarina 

 (in which genus the " intermediate skeleton " attains a development that is unexampled else- 

 where), the connection of which with the cavities of the chambers is so direct, that it may be 

 said to be nothing else than a prolongation of these into the parts of the fabric most removed 

 from them. \n Dacfi/Iopora, indeed, such connection as it has with the chambers seems to be 

 rather accidental than essential, resulting from the fact that the "junctural canals" and the 

 apertures of the chambers happen to coalesce near to the point of their common connection 

 with the general cavity of the cylinder. And it seems to me that we cannot form any other 

 rational conception of the animal of this curious organism, than by regarding it as an aggre- 

 gate mass of sarcode, occupying the whole cavity of the cylinder, and in connection, on the 

 one hand, with the segments that occupy the chambers, and on the other with the extensions 

 that radiate past them through the exogenous sheath beyond. 



198. Affinities. — Looking at the most complex and specialized forms of Bactylopora, we 

 should not easily recognise in them any but a very remote affinity to either of the porcellanous 

 types hitherto described. But the true "idea"' of its structure is to be looked for in its 

 sin^ler forms ; and, as already pointed out, there is much in the structure of B. annuhs that 

 reminds us of that of a single annulus of Orhitolites. If, in fact, the segments were united by 

 internal continuity instead of by external, and if the new rings were developed from the 



